1. Field of the Invention
The present invention relates in general to an electroluminescence (EL) device having an EL element, and relates in particular to an EL device, EL display apparatus, EL illumination apparatus having improved frontal brightness, and liquid crystal and electronic apparatuses based on these apparatuses. The present invention is based on Japanese Unexamined Patent Applications, First Publications, Numbers 2001-164896, and 2001-167308), the contents of which are incorporated herein by reference.
2. Description of the Related Art
One application of an electroluminescence (EL) device known in this field is in EL displays using EL elements.
In the following, an outline of a conventional EL display apparatus will be given with reference to the drawings.
In the following, the surface at which light exits, or at which light would exit if it did not encounter the face at an angle not less than the critical angle is called the “exit face”.
FIG. 31 shows a schematic cross sectional view of a conventional display apparatus.
The conventional display apparatus is comprised primarily of a transparent substrate 801, and an EL element 812 provided on one of the surfaces of the transparent substrate 801.
EL element 812 comprises indium tin oxide (referred to as ITO below), etc., and includes successive layers deposited on the transparent substrate 801, starting with a transparent electrode 813 serving as an anode, followed by a bole transport layer 814 for facilitating injection of holes from the transparent electrode 813, a light emitting layer 815 comprising an EL material, and finally, a metal electrode 816 serving as a cathode, so that the transparent electrode 813 opposes the metal electrode 816 across the light emitting layer 815. An EL display apparatus having such an EL element 812 operates by flowing electrical current of a specific magnitude to the transparent electrode 813 and the metal electrode 816 to emit light from the light emitting layer 815 so that the light from the light emitting layer 815 is transmitted through the transparent electrode 813 and the transparent substrate 801 to emit light from the transparent substrate 801 side to the external environment of the EL element 812.
However, of the light emitted from the light emitting layer 815, the light passing through the transparent electrode 813 and forming a wide angle (an angle not less than the critical angle) when at the exit face of the transparent electrode 813, and also, the light passing through the transparent electrode 813 and passing through the transparent substrate 801 which forms a wide angle (an angle not less than the critical angle) when at the exit face of the transparent substrate 801, undergo repeated internal reflection in the transparent electrode 813 and the transparent substrate 801, respectively, and are not emitted externally from the transparent substrate 801. That is, the light which enters the transparent electrode 813 and transparent substrate 801 and forms a wide angle (not less than a critical angle) when at the exit face thereof is not utilized for display purposes, resulting in low brightness of displayed images.
It should be noted here that when a light from a medium having a refractive index n1 is incident at an angle θ1, and travels into a medium having a refractive index n2 at an angle of refraction θ2, the following relation (Snell's law) is established among the parameters θ1, θ2, n1 and n2:n1 sin θ1=n2 sin θ2.
The critical angle of light emitted from the light emitting layer and injected into the transparent substrate 801 is obtained by knowing that, for transparent substrate 801, n1=1.49 to 1.6 for glass plate or acrylic resin, which are normally used for this purpose, and that the refractive index of air is 1 so that n2=1, and incident light entering at the critical angle produces transmitted light parallel to the surface of the transparent substrate 801, so that the angle θ2 between the normal direction H and the transparent substrate 801 is 90 degrees, resulting in θ1 of about 40 degrees, and therefore, the critical angle of the light entered into the transparent substrate 801 at the exit face is about 40 degrees.
Therefore, in the conventional EL display apparatuses, this problem is solved as shown in FIG. 31 by forming a scattering layer 820 having isotropic scattering characteristics and a thickness of 50 to 200 μm, by the use of a substrate such as triallylcyanate and a filler of metallic oxide particles dispersed therein, at a surface opposite to the side of the transparent substrate 801 having the EL element 812, so that even when the light injected into the transparent substrate 801 (the light emitted from the light emitting layer 805 and injected into the transparent substrate 801) forms an angle not less than the critical angle at the exit face, it can be transmitted to the external environment of the transparent substrate 801.
However, a problem in the conventional EL display apparatus having such a scattering layer 820 described above is that, although it is possible to output the light L10 injected into the transparent substrate 801 forming an angle not less than the critical angle at the exit face by utilizing isotropic scattering effects (transmitted light L11 is scattered isotropically) to the external environment of the transparent substrate 801, as shown in FIG. 31, isotropic scattering occurs also for the light L12 injected forming an angle less than the critical angle (isotropic scattering of transmitted light L13) at the exit face, resulting in that although brightness is maintained over a wide viewing angle, brightness is low in the frontal direction (normal direction and its vicinity) so that the display images appear dark.
Another method for outputting the light emitted from the light emitting layer and injected into the transparent substrate 801 forming an angle not less than the critical angle at the exit face for delivery to the viewer is to roughen the surface of the transparent substrate 801 to provide concavities and convexities, but in this method also, as in the case of the problem caused by the above scattering layer, brightness in the frontal direction is decreased and the displayed images appear dark.
Also, it has not been possible in the conventional EL display apparatus, having roughened surface on the transparent substrate 801 or having a scattering layer 820, to prevent those rays emitted from the light emitting layer 815 and injected into the transparent electrode 813 to form a wide angle (not less than the critical angle) at the exit face, from causing total internal reflection within the transparent electrode 813.